Okay kiddo, so Poland is a country in Europe that had some bad things happen to it during World War II. "Betrayed" means that someone you thought was your friend or helper did something really mean to you.
During the war, Poland was invaded by Germany, who wanted to take over their land. But before the invasion, Poland had signed treaties with other European countries promising to help each other if they were attacked.
Unfortunately, those other countries didn't really do much to help when Germany invaded, and Poland was left all alone to fight. That's why some people say that Poland was "betrayed" by its allies.
The invasion was really bad for Poland. Lots of people were killed, and the country was occupied by Germany for many years. It was a really tough time for the Polish people.
So when someone says they "saw Poland betrayed," they mean that they saw how other countries didn't keep their promises to help Poland during World War II.